1. Field of the Invention
The present invention relates to a sensor of weak magnetic fields, with magnetoresistive effect, and more specifically to the structure of the layers of materials that constitute such a sensor.
The term "sensor with magnetoresistive effect" is used to designate a sensor using the variation of resistivity of at least one material that forms it as a function of the variation of a magnetic field. A weak magnetic field is the field radiated by the tape of a magnetic recording for example.
2. Description of the Prior Art
Various types of sensors with magnetoresistive effect are known and are implemented in systems for the reading of magnetic recordings such as tapes, drums and diskettes. These sensors comprise a monolithic part, a block made of a ferromagnetic alloy with high magnetoresistance, which is placed in the magnetic circuit of a reading head which reproduces the variations of the electrical resistance of the magnetoresistive sensor while a recording flows past. The alloys most frequently used, at present, are nickel-based ferromagnetic alloys such as NiCo, NiFe (Permalloy) which have high magnetoresistance but which, at ambient temperature, give a relative variation in resistance of only a few percent.
Sensors of greater sensitivity have been developed more recently: they use a phenomenon known as "giant magnetoresistance" which develops when the sensitive element of the sensor is constituted by a multilayer of alternating magnetic and non-magnetic metals. In this multilayer, the magnetic layers have ferromagnetic (parallel) type coupling, outside any magnetic field, or anti-ferromagnetic (anti-parallel) type coupling under a magnetic field, and the transition from one state to the other occurs on a small interval of a field. The drawback of these multilayer sensors is the excessive value of the magnetic field needed for the "reversal" to make the magnetizations of the layers transit from the parallel state to the anti-parallel state. A strong magnetic field is needed when the layers have regular thicknesses and are coupled to one another. The energy of the "reversal" is equal to the sum of the energies used for the coupling of one layer with the two adjacent layers.